A Possible Common Role for DNA Polymerase I and Exonuclease V in Escherichia coli

Abstract

In an attempt to construct the double mutant polA1recB21, which is thought to be inviable (Monk and Kinross, 1972), we have transduced JG138 polA1thyA with phage P1 grown on AB2470 recB21thy+ and selected thy+ transductants. Among the transductants were some strains of the type polA1recB21sbcA (Strike and Emmerson, 1972), which had acquired an ATP-independent DNase characteristic of recB mutants suppressed by sbcA mutations (Barbour et al., 1970). Since it is extremely improbable that JG138 could have acquired both recB and sbcA mutations simultaneously by transduction it is likely that an intermediate strain polA1recB21 was involved. On further examination of small transductants some apparently unsuppressed polA1recB21 strains were also found. Fig. 1 shows the results of assaying lysates of one such strain, PE114, for DNA polymerase I, Exonuclease I and Exonuclease V activity. This strain lacks DNA polymerase I, Exonuclease V and the ATP-independent DNase characteristic of sbcA mutants. Moreover, since it has normal levels of Exonuclease I it does not carry an sbcB suppressor mutation which could also suppress recB (Kushner et al., 1971). Thus, PE114 does not appear to have an amber suppressor of polA1 or an indirect suppressor of recB such as sbcA or sbcB. Only about one per cent of the viable cells in a culture of PE114 are able to give rise to colonies compared with about ten per cent in the case of recB21 single mutants. We conclude therefore that polA1recB21 double mutants are viable although very unhealthy and prone to reversion.